RE: ASTM A563 apparent incongruity

When re-used, a few of the "bad" ones loosened in service. Repeatedly. Brand New commercially available "heavy hex" nuts have been working fine.
The one "bad" one available is HRB65, just a couple of points below the ASTM A 563 minimum of HRB 68. The grain structure is pretty coarse. The threads don't appear "pulled" or deformed in any way, other than wear/erosion on the first couple of threads. The lone "good" used nut is HRC 30, with a much finer grain structure, as if quenched and tempered. Despite those differences between the two nuts, the cause of The self loosening is kind of a mystery. There are lots of things about the very mature design that I don't like, but apparently it normally works just fine.

I posted because What really hangs me up, looking strictly at the ASTM spec in the previously attached image, is how a Grade A nut with the required minimum proof stress load could possibly also have the low allowable minimum hardness.

RE: ASTM A563 apparent incongruity

If I'm not mistaken you normally want to have nut hardness somewhat less than that of the stud/bolt, but 68 HRB minimum seems extremely low. Also, the range between B68 and R32 is very wide. It seems like ASTM set some arbitrary lower limit not quite reflective of reality.
For critical fastener applications I advise clients to set up a QC regimen where they sample fasteners for hardness testing, magnetic particle examination, thread profile inspection, and metallographic sectioning to look for structure, decarburization, and forming defects such as laps and cracks. Then segregate the lots these tests represent. Class of fit is another criterion to pay attention to.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."